The objective of this project is to build and evaluate a precision, microprocessor controlled gas mixer. The mixer will provide gas at a constant flowrate with a controlled concentration of up to four component gases. The mixer design is based on the relationship of mass flow to differential pressure across an orifice with fixed area and discharge coefficient. For each gas channel (total of four channels) gas flow is monitored by measuring the differential pressure across a calibrated orifice. The actual flow is fed back to the microprocessor and compared to the desired output concentrations inputed by the operator. Corrections in gas flow are obtained by repositioning the stepper motor driven flow control valve. Correction factors are stored in the microprocessor to allow accurate control of a wide variety of gases. Phase I will address design feasibility and investigate application to various medical and anesthesiological problems.